To transport cargo on a truck, railcar or other vehicle, a cargo bed structure is needed that will bear the weight of the cargo during transport. The cargo may comprise regular shapes such as rectangular crates and the like, which are stackable, or may comprise items that do not stack in a stable manner, such as cylindrical items. Structure is needed to confine such unstable items to the cargo bed. Examples of elongated unstable load items are logs, pipes and the like. Such items typically (although not exclusively) are arranged parallel to the longitudinal axis of the vehicle, and are stacked in ranks atop one another. Insofar as the load is carried on a flat cargo bed or a bed having members disposed in a plane, some sort of lateral confinement is needed to keep such unstable items in the stack rather than permitting the items to roll and thereby spread laterally out from a compact stack.
Various techniques are known for supporting and laterally confining loads. Some cargo holding structures, such as open-top box structures, may be employed to support load items without regard to whether or not the load items are elongated, whether or not they are shaped to form stable stacks, etc., because the box shape holds the contents, having vertical walls bordering and confining the load to a flat bed. For loads that extend longitudinally, a continuous flat bed may not be required and longitudinally spaced lateral beams can support elongated load items that bridge between at least two adjacent beams. Unstable loads such as logs or pipes are supported on a continuous flat bed or on longitudinally spaced lateral beams, and the load is confined laterally. The lateral confinement can involve longitudinally extending sidewalls or a series of spaced vertical posts along the lateral edges of the flat bed, often referred to as stakes. Other possibilities are possible such as flexible chains or belts that wrap over and are cinched to confine the load. Similarly to the stakes or stake receptacles provided on many flatbed trucks, a truck or other support vehicle having longitudinally spaced lateral beam members or bunks can also have vertical structures resembling stakes, mounted at the lateral ends of the bunks.
Prior art trucks that are used in connection with the hauling of unstable or loosely stacked loads such as raw timber, conduit pipe, etc., normally have a chassis fabricated from two or more steel I-beam members spaced apart laterally, and extending from the front to the rear portion of the truck's cargo bed. A plurality of shorter, transversely extending beam members, known as bunks, are supported by the twin I-beams in longitudinally spaced relation to one another. The bunks support the load at spaced locations and distribute the weight evenly along the I-beams. The bunks are secured to the I-beams by bolts, welding or the like. Such cargo bed constructions are well known in the art and are suited for transporting elongated cargo items that span over two or more adjacent spaced bunks. See, for example, U.S. Pat. Nos. 5,580,121; 5,567,096; 5,433,055; 5,071,164; 4,9899,768; 4,623,161; 4,084,835; 3,999,354; 3,972,540; and 3,944,257.
Inasmuch as elongated items can be supported on spaced bunks as opposed to a full bed having structures extending continuously over the full length and width, the transverse bunk supporting structure can results in a saving of overall weight in the vehicle. The bunks and I-beams require sufficient structural strength and rigidity to support the elongated items. A particularly demanding example of elongated load items to be transported is a load of rough cut timber. Such a load may comprise a plurality of large and heavy solid wood logs. In loading the logs expeditiously in field conditions, it is not unusual that the logs are set down hard on the transport vehicle, or even dropped onto the cargo supporting bed. The structural strength required to survive such treatment can be achieved by use of I-beams that are thick enough to bear the load. Such I-beams comprise an elongated central web that is typically oriented in a vertical plane, integrally coupled to upper and lower parallel webs that are perpendicular to the central web and typically are oriented horizontally to provide horizontal support or support-engaging surfaces, respectively. The tendency of an I-beam arranged in this manner to bend due to a vertical load is resisted because bending force is applied in the plane of the central web. However the I-beam must nevertheless be thick and durable enough to withstand the expected loads, and therefore can be quite heavy. Solid bars or laminations of elongated solid plates can be mounted or attached to one another to form solid or semi-solid longitudinal beams and lateral bunks. Solid or semi-solid members of this type can provide the necessary rigidity and resistance to bending and warping, but they are heavy, adding considerably weight to the vehicle. The vehicle has a maximum gross weight, both in terms of design weight and maximum legal weight for over-the-road travel. Therefore, any unnecessary weight caused by particularly durable or unnecessarily heavy structural members reduces the weight capacity available for carrying cargo on the vehicle. For a log truck, added weight means that fewer logs can be carried per load, and more trips are required to move a given number of logs.
There is a need for a vehicle chassis, especially for carrying elongated cargo items such as logs, that provides the structural strength needed to support a load of heavy elongated items, preferably at longitudinally spaced points, while at the same time having the least weight that will reasonably conveniently provide sufficient load bearing strength to carry the load without deformation to the vehicle.